This invention relates to an optical fiber for use as a transmission line in optical communications.
A fiber heretofore proposed for optical transmission is a glass fiber which is formed of glass consisting principally of silicon dioxide (SiO.sub.2) and has a concentric structure comprising a core for propagating light and a cladding of a little lower refractive index than the core. The glass fiber for optical transmission is divided into a quartz glass fiber in which additives such as GeO.sub.2, P.sub.2 O.sub.5, B.sub.2 O.sub.3, etc. are contained in quartz glass of the principal component for the purpose of changing its refractive index, and a multi-component glass fiber which contains several kinds of components such as alkali oxides in addition to SiO.sub.2. In either case, the SiO.sub.2 glass is used as a principal material. Of the conventional fibers, although the quartz glass fiber has the smallest transmission loss, the transmission loss is not so reduced in practice in the long-wavelength band.